Parkinson's disease (PD) is characterized primarily by the loss of dopaminergic neurons in the substantia nigra that causes functional alterations of basal ganglia and typical motor symptoms. Dopamine replacement therapy has beneficial effects in the early stages, but as disease progresses, dopaminergic treatments fail to restore normal mobility and even produce additional motor abnormalities. Such altered responses to dopamine have been related to complex changes of dopamine D1 and D2 receptor-mediated mechanisms that regulate the striatal direct and indirect outputs. However, the activity of striatal projection neurons is also regulated by other neurotransmitter systems whose mechanisms may undergo plastic changes in the chronic evolution of PD. The glutamate system provides abundant cortical and thalamic excitatory inputs to the striatum, and several lines of evidence indicate that the increase of glutamatergic signaling may contribute to striatal dysfunction. This project is intended to study the role of dysregulation of striatal glutamatergic transmission in the pathophysiology of abnormal dopamine responses in chronic PD. The ultimate goal of the project is to identify targets for developing new treatments for the long-term management of PD. Specifically the project comprises three aims: 1. to study the relationship between glutamatergic hyperactivity and altered discharges of striatal projection neurons in chronically parkinsonian monkeys. We will use different classes of glutamate antagonists to block the striatal receptors locally with drug injections into the brain. 2. To examine the contribution of excessive glutamate release in the mechanisms of altered striatal discharges in the chronic primate model of PD. We will use cannabinoid CB1-acting drugs in striatal injections or infusions to study the effects on striatal neurons and motor responses to levodopa. 3. To study the role of striatal glutamatergic hyperactivity on changes in dopamine responses driven by the indirect striatal output pathway in the chronic primate model of PD. We will use glutamate antagonists in striatal infusions to examine the responses to selective dopamine agonists in external pallidal neurons. The project combines modern technologies in pharmacological and electrophysiological areas to study the underlying mechanisms of striatal dysfunction in chronic PD, and thus, it may contribute to developing new therapies for patients debilitated by the advanced disease.